1. Field of the Invention
The present invention relates generally to a coating method for preparing a light absorbing layer of a solar cell, and more particularly, to a coating method adapted for being executed in a non-vacuum environment.
2. The Prior Arts
Typically, solar cells can be categorized into monocrystal silicon solar cell, polysilicon solar cell, amorphous silicon solar cell, dye-sensitized solar cell, and copper/indium/gallium/selenium (CIGS) solar cell, and so on. Comparing with the silicon solar cell which relies on the supply of silicon wafers, and the dye-sensitized solar cell which employs specific and expensive sensitizing dye, a CIGS solar cell is featured with outstanding advantages. For example, the CIGS solar cell is made of copper, indium, gallium, selenium which are much cheaper than the raw materials of the silicon wafers, as well as the sensitizing dye. Further, the CIGS solar cell can achieve a photoelectric conversion efficiency, up to 20% to 30%, and even a CIGS solar cell formed on a flexible plastic substrate can also achieve a photoelectric conversion efficiency, up to 14%. As such, CIGS solar cells are believed a very promising kind for further development.
A typical CIGS solar cell mainly includes a Cu(InGa)Se2 layer, and a ZnS layer, serving as a P type layer and an N type layer, respectively. A P-N junction is formed at an interface between the Cu(InGa)Se2 layer and the ZnS layer. The Cu(InGa)Se2 layer is coated on a molybdenum layer which serves as a back electrode, and the molybdenum layer is formed on a glass substrate.
Currently, a series of vacuum processing procedures developed by Shell Solar Inc. (SSI) are mainly commercially used for fabricating CIGS solar cells. However, such vacuum processing procedures must be performed with very expensive vacuum equipment, which is complicated and difficult to maintain.
Another conventional technology proposes to execute a coevaporation process or a selenization process for configuring the Cu(InGa)Se2 layer. According to a coevaporation process, individual evaporation sources, e.g., Cu, In, Ga, Se targets, respectively, are heated to evaporate at the same time, so as to form a Cu(InGa)Se2 layer on the molybdenum layer. Specifically, the Cu target is heated to a temperature of 1300° C. to 1400° C., the In target is heated to a temperature of 1000° C. to 1100° C., the Ga target is heated to a temperature of 1150° C. to 1250° C., and the Se target is heated to a temperature of 3000° C. to 3500° C. However, such a coevaporation process is hard to control. Specifically, the evaporation amount of the Cu target is not easy to precisely control.
As to the selenization process, it employs two step processing, in which Cu, In, and Ga are sputtered to deposit on a substrate to form a precursor film, and then selenium hydride is added thereto to react with the precursor film, thus obtaining the Cu(InGa)Se2 layer. However, the selenization process has a low freedom of controlling the ingredients, and is hard to vary the energy gap thereof, and the produced Cu(InGa)Se2 layer film is featured with a poor bondability to the substrate. Accordingly, both of the coevaportation process and the selenization process are remained at a laboratory phase, and not yet be commercially developed.
Accordingly, a method for preparing a Cu(InGa)Se2 layer having a high reliability and an improved photoelectric conversion efficiency under an atmospheric pressure is highly demanded.